Spider spinneret, coloured scanning electron micrograph (SEM). This structure, one of several, is found on the tip of a spider's abdomen. It produces the silk which spiders use for a wide variety of purposes, such as safety lines, webs, and shelters. Some spiders also use silk to spin cocoons for their eggs. Spider's silk is made of a protein polymer called fibroin. It is very elastic and can be stretched 30-40 percent of its length before it breaks.